Are plantain allergies the same as banana allergies?

Not usually: Usually the plantain allergy discussed here in the states is to the pollen of the weed English Plantain, not food allergy to plantains (similar to bananas). An interesting fact about English Plantain is that, unlike typical weeds like ragweed that produce pollen in the fall, English Plantain produces pollen both fall AND spring, in a bimodal fashion. This photo is not E. plantain, however.

May be: Plantain and banana are from the same family so it is possible to have allergy to both. However, plantain is cooked before it is eaten and could have denatured the allergent during the cooking process. Many people that are allergic to raw apple can eat apple pies (cooked) for the same reason.